The present invention concerns an injection pump for installations of hot-chamber die casting of light alloys which are corrosive in the molten or semi-liquid state, and in particular an injection pump equipped with static sealing elements that are non-metallic and yield under compression.
In injection pumps for the die casting of molten non-ferrous alloys, for the construction of the injector piston, of the container cylinder and of the dynamic sealing elements arranged therebetween there are currently employed materials resistant to high temperatures, able to withstand the corrosive action of the alloys in the molten or semi-liquid state, to resist the wear generated by the friction of the components in relative motion and to prevent or hinder the possible seizures, as well as to contain the leakage of alloy in order to obtain an economically acceptable service life of these components.
In the so-called “hot-chamber” installations, in which the injection pump is immersed in the molten alloy, there are generally used dynamic sealing elements in the form of elastic metal rings applied coaxially to the injector piston. The elastic rings have a cut in the circumferential direction that allows them to compress and expand elastically in the radial direction and are mounted coaxially to the injector piston in suitable grooves or circumferential seats formed in its lateral surface. During assembly of the injector piston in the container cylinder, the elastic rings are compressed in their seats closing on the injector piston and subsequently expand by virtue of their elasticity going in contact with the internal surface of the cylinder. The pressure generated by the sealing elements on the walls of the container cylinder determines the degree of sealing against the injection pressure of the alloy being processed, and depending on the conformation of the cut in the circumferential direction and on the conditions of operation of the injector piston (e.g. pressures and speeds) one or more elastic sealing rings can be provided.
The friction between the internal surface of the container cylinder and the dynamic sealing elements integral with the injector piston, moving fast during the injection of the molten alloy, generates considerable problems of wear of these components which cause over time defects in the sealing against the pressure and imply frequent and costly maintenance. Since the friction and wear between surfaces in relative sliding depend on the characteristics of the materials and of the coupled surfaces, as well as on the dynamic characteristics of the motion especially in terms of pressure and relative velocity, the most appropriate pairings as to shapes and materials of the components in contact are subject to continuous research and trials with the aim of maximizing their useful life.
The pairings between non-metallic materials and the mixed pairings between metallic materials and non-metallic materials are currently deeply studied and generally the wear is directed on the surfaces of the components that are easier and cheaper to replace, for example, in the case of injection pumps for die casting, the sealing elements between the container cylinder and the injector piston. However, despite numerous studies in the field (see e.g. JP 55088966), the state of the art does not yet offer a satisfactory solution to the problem of obtaining wear components with an economically acceptable service life combined with good sealing ability against pressure.
In particular, prior art pumps are not suitable for the die casting of aluminum alloys because of their very high corrosiveness, in the molten or semi-liquid state, on the metallic elements of the pumps and of their connections with the molds. For this reason, aluminum alloys are die cast only with the “cold chamber” process in spite of the undoubted advantages of the “hot chamber” process that allows to:                inject the alloy at the same temperature of the bath that feeds the pump with temperatures, speeds and pressures much lower than the “cold chamber” process, therefore with less energy consumption and less wear of the molds;        control the process in a closed cycle, resulting in increased productivity, improved product quality with strict repeatability of their characteristics, less rejects and less consumption of raw material;        obtain a better microstructure of the castings without the oxidations and inclusions of gas typical of the “cold chamber” process, therefore with the possibility to weld the castings between them and with other structures and to have a better sealing towards compressed gases, in addition to expanded possibilities of heat and galvanizing treatments;        use smaller and cheaper molds and presses;        inject into the same mold, simultaneously or sequentially, alloys with different characteristics thus obtaining monolithic castings with component parts which greatly improve the qualities and overall performances of the casting itself, usually not obtainable from a single alloy, such as wear resistance, lightness, workability, tensile strength, impact resistance, corrosion resistance, etc.        
Proposed arrangements for hot-chamber pumps for aluminum alloys are known in the art, some for decades, and some among them have been experienced by large industries, but none has entered so far in the production technology. They are usually based on ceramic components that are resistant enough to corrosion by molten aluminum, but insurmountable limitations are currently found in the poor resistance to tensile and bending stresses of these components, as well as in the high fragility and in the limits of production as to their size, shape, workability and difficulties in the connections to the metal structures of the presses, with negative consequences on the costs and risks in manufacturing and operating the pump. Similar pumps with ceramic components have been proposed, for example, by Miki Isao in EP 0827793 and by Yuji Ogawa in JP 2008006455 and JP 2008073698.
As alternative solutions, the applicant had proposed in U.S. Pat. No. 5,385,456 to make irrelevant the corrosion of the steel cylinder by using a plunger piston, possibly ceramic, subject only to compression forces, resistant to corrosion and equipped with non-metallic seals. However, this pump configuration required the use of a feed valve, which was a weak point of the solution. The applicant considered to overcome the problems of the previous configuration by proposing in U.S. Pat. No. 6,029,737 a plunger piston with grooved end, subject only to compression forces and provided with an automatic rigid seal, which has shown, however, functional problems in the pilot plant and major difficulties of maintenance.
In another alternative configuration, the injector piston can slide inside a liner in turn inserted into the body, so that when the internal surface of the liner is worn it is sufficient to change the liner instead of the entire body. For reasons of manufacturing cost, such a pump has the body made of a steel suitable to resist the tensile stresses at high temperature and with both the internal and external surfaces protected with coatings resistant to corrosion by molten aluminum as, for example, sprayed ceramic powders and binders, or other barrier layers known in the art.
For functional reasons, the steel body must be coupled to a liner which is resistant to the high tensile stresses; of a strongly impulsive nature, required by the process. The internal surface of the liner must resist the sliding of the injector piston and the corrosion by the molten alloy in order to ensure, for an economically acceptable time, the generation of the pressures required by the process of filling the mold. This liner must be easily removable from the pump body for routine maintenance, reconditioning and replacement, being able to tolerate wear of an order of magnitude lower than that of the body. Also its external surface must resist the corrosion by the molten alloy and simultaneously there is required a coupling, resistant to corrosion and high pressures, between the external surface of the liner and the pump body.
The constituent materials of the liner, which is a substantially cylindrical sleeve, may be of ceramic nature, resistant to compression stresses, or metallic nature, resistant to both compression and traction stresses. For the ceramic option some advanced ceramics have been proposed, such as e.g. silicon nitride, while for the metallic option alloys of heavy metals with high melting point have been proposed, such as molybdenum and tungsten, whose surfaces can be hardened to withstand the sliding wear as described, for example, in IT 1376503.
Whatever the material chosen for the liner among the various options above, it will have a much lower thermal expansion than the steel of the body, since making the whole pump body with the same material of the liner would entail a prohibitive cost. This makes impossible the direct coupling between the liner and the pump body according to the solutions traditionally known in the art such as, for example, the interference fit between the parts, given that the spaces generated by the greater thermal expansion of the internal surface of the body compared to the expansion of the external surface of the liner would cause intolerable leaks of molten alloy, which would prevent a proper filling of the mold cavity.
To overcome these problems various solutions have been proposed, generally referable to a frontal, flat or conical contact (e.g. U.S. Pat. No. 6,029,737) between the ceramic or metallic organs, with or without interposition of seals between the surfaces.
For example, DE 1583714 describes a solution with a gasket of expanded graphite arranged between the liner and the pump body, with a flat contact in a first embodiment and a conical contact in a second embodiment. In both cases, the pressure exerted from below on the seal by the molten alloy is countered only by an upper flange of the liner which is in turn pushed upwards by the molten alloy and held in position by an element screwed to the pump body. Due to the high cyclical injection pressures the pump body elongates and the element screwed to it shortens, moreover due to the high temperature the forces generated by the screws are very modest. It follows that the sealing effect of the gasket decreases with increasing pressure and temperature of the molten alloy, with the risk of leaks which can erode and destroy the gasket.
JP 03110056 describes instead a solution in which the seal between the liner and the pump body is provided by a pair of reverse conicity bushes arranged between said two elements without the interposition of any gasket, other three concentric elements push from above respectively the liner and the two conical bushes under the action of an upper plate. The molten alloy is fed into the space between the innermost pushing element and the intermediate pushing element, which is internally provided with a coating of graphite, ceramic or other material resistant to the molten alloy.
None of the solutions proposed so far led to the industrial development of the project due to the rapid deterioration of the sealing surfaces.